Integrated circuits are chemically and physically integrated into a substrate, such as a silcon wafer, by patterning regions in the substrate, and by patterning layers on the substrate. Deposited conductors are an integral part of every integrated circuit, and provide the role of interlevel and surface wiring for conducting electrical current. Specifically, the deposited conductors are used to wire together the various components that are formed within the wafer. Such conductors are commonly known as "lines" or "runners". Conductors also provide other functions in integrated circuit structures, such as fuses and back side electrical contacts for the packaged die.
Common materials for forming conductive lines or runners are elemental metals and metal alloys. Metal material would be applied atop the wafer, by techniques such as sputtering or chemical vapor deposition, and thereafter photomasked and etched to produce the desired construction. Unfortunately, many metals do not have the same expansion/contraction properties (for example, thermal expansion coefficient) of the underlying substrate, which is predominantly either silicon or silicon oxides.
Internal and external mechanical stresses are associated with applied metal films. Internal mechanical stresses are inherent in any applied solid material and relate to stress which develops upon temperature change of a material which leads to expansion or contraction. External stresses result from the difference in expansion properties of two adjacent materials which form an adhesive bond relative to one another. Any relief of such mechanical stresses is desirable to minimize any tendency of separation between layers or fracture. High stress in metal films is particularly a problem with chemical vapor deposited tungsten and titanium nitride films in current metalization semiconductor processing technology.
Internal metal film stress is typically not of significant concern with respect to sputtered metal films. Sputtered metal films inherently exhibit poor step coverage over the varying typography on a semiconductor wafer. To alleviate this, sputtered metal films are melted causing reflow of the material to produce adequate step coverage. The step of melting and completely filling the valleys on the wafer is known within the art to result in an inherent relieving of internal mechanical stresses which exist in the as-deposited sputtered film.
On the other hand, chemical vapor deposited films exhibit excellent step coverage, requiring no subsequent melting or reflow. High temperature furnace annealing has shown promise in reducing internal and/or external stresses associated with chemical vapor deposited metal films. For example, treatment of chemical vapor deposited tungsten above 700.degree. C. has shown an ability to reduce the magnitude of the inherent stress, particularly tensile stress, and also to change the sign of the particular stress from compressive to tensile or vice versa. Thus, film stress in metal layers can apparently be reduced by proper high temperature annealing.
This invention concerns alternate methods of reducing mechanical stress in chemical vapor deposited metal films.